There are several limitations of conventional ultrasound imaging systems that result in reduced spatial and temporal resolution in generated ultrasound images. One limitation is that conventional systems do not offer much flexibility in controlling imaging parameters on an intra-frame or regional basis. For example, many imaging parameters such as filtering, operating frequency, gain, number of transmit foci, and line density are uniform across the image frame, although line density may follow a predetermined reduction toward the sides of the image. Other imaging parameters may be controlled on a regional basis in a very limited manner. For example, gain in the axial or the lateral imaging dimension can be controlled in a region by using slide potentiometers for each depth in the image. The Regional Expansion.RTM. feature and the zoom feature found in commercial imaging systems also provide a very limited amount of regional control. The Regional Expansion.RTM. feature increases line density within a region to enhance spatial resolution, but the area outside the region is not scanned. The zoom feature merely magnifies a region, typically to fill the display screen. Additionally, modes such as color flow velocity, energy, or combined energy and velocity modes with a B-mode display can operate in a particular region of the image.
Another limitation of conventional ultrasound imaging systems is that they typically suffer from low frame rates. Because ultrasound systems typically present one image for every set of lines acquired, there is a tradeoff between spatial resolution and frame rate. That is, if a system is performing deep, detailed imaging requiring, for example, high line density and multiple transmit foci, a greater amount of time is required to acquire a set of lines. This increased amount of time can result in a frame rate that will be uselessly low. For example, with 256 lines, an imaging depth of 300 mm, and 5 transmit focal zones, the frame rate is merely two frames per second. In many areas (such as cardiology), a severely compromised frame rate is unacceptable. While repeatedly displaying the generated frames can match the video refresh rate (30 frames per second in the United States), repeated frames provide no new information to the user.
A third limitation of conventional ultrasound imaging systems is due to the complexity of spatial and temporal controls. Typically, a user quickly scans a patient to find a particular area of interest and then slowly scans that area to acquire a detailed image. Although the optimal imaging parameters are different in a fast scan than in a slow scan, many users choose not to fully optimize these parameters because adjusting imaging controls is often cumbersome.
A fourth limitation concerns transducer overheating. With some imaging modes, such as Color Doppler, the transducer will reach an elevated temperature if, while the ultrasound system is powered, the transducer is not used for imaging. Elevated temperatures can be damaging to the transducer and have been implicated as a contributing factor to reduced probe life. Elevated temperatures are also undesirable to a patient if a hot probe is applied to the patient. Some approaches to solving this problem include attaching a magnetic position sensor or an accelerometer to the transducer probe to automatically turn off the probe when no motion is sensed--an indication that the probe is not in use. These sensors are typically expensive (partly because they offer more features than are needed to solve the problem) and require modifications to be made to the probe. Other methods involve using latched probe holders, but to be effective, these methods require the user to place the probe in the proper holder. There is, therefore, a need for an inexpensive alternative that does not require user intervention.
A fifth limitation of conventional ultrasound imaging systems is that the displayed image typically exhibits geometric distortions due to image or transducer motion during the acquisition of an image frame. Scan lines in an ultrasound frame are acquired sequentially--not simultaneously. Accordingly, a finite amount of time elapses between the acquisition of the left-most line and the right-most line in an image frame. Image or transducer motion after the ultrasound system acquires the left-most line but before it acquires the right-most line can result in a distorted image. Distortions can also be caused by high transmit focus zone formats and in the mixed B-mode/Color Flow modes where there is a significant time delay between B-Mode lines acquired on the left and right hand side of the color box. An additional distortion occurs when transducer motion results in a scan line being fired at a physical location that corresponds to a location outside the image frame.
There is, therefore, a need for an ultrasound system and method that will overcome the problems described above.